Higher organisms progress through several discrete developmental stages during their post- embryonic development. The molecular mechanism of these transitions, and the endogenous and environmental factors that influence their timing, are major problems in developmental biology and have important implications for human health. Molecular-genetic analyses of the juvenile-to-adult transition in plants and Caenorhabditis elegans have revealed striking similarities between these processes. In both plants and C. elegans, stage transitions are regulated by a change in the abundance of miRNAs that repress the expression of transcription factors or other key regulatory proteins. However, the molecular basis for this change miRNA levels, and the effect of this change on the expression of the direct targets of these miRNAs are still incompletely understood. In Arabidopsis thaliana, the juvenile-to-adult transition (vegetative phase change) is regulated by a decrease in the expression of two related miRNAs, miR156 and miR157 that promote the expression of the juvenile phase by repressing the expression of SPL transcription factors. The factors responsible for the temporal expression of these miRNAs will be identified using a directed molecular approach and a sensitized genetic screen. To determine the regulatory architecture of the pathway that regulates vegetative phase change, the function of the SPL genes regulated by mir156/miR157 and the ways in which they interact with each other and with MIR156A and MIR156C will be determined by characterizing the developmental and molecular phenotypes of loss-of-function mutations in these genes. The quantitative effect of miR156 on the expression of its direct targets will be measured using protein-reporter constructs. This will reveal if miR156 fine-tunes gene expression, or acts as a thresholding factor to promote alternate developmental states. Finally, the organismal location of the developmental clock that regulates vegetative phase change will be investigated by testing the role of the shoot apical meristem in this process. These experiments will provided detailed information about the molecular mechanism of vegetative phase change in Arabidopsis, and contribute to an improved understanding of the regulation of developmental timing in higher organisms.